1. Field of the Invention
The present invention concerns a reflective type color liquid crystal device and an electronic apparatus using this.
2. Prior Art
The display mounted on a portable information terminal first must be low in power consumption. Consequently, reflective type liquid crystal devices not requiring backlights are optimal for this purpose. Nevertheless, the conventional reflective type liquid crystal device was mainly a monochrome display, and a good reflective type color liquid crystal device is yet to be obtained.
The development of the reflective type color liquid crystal device appears to have been started in earnest from the middle of the 1980s. Before that, for example, as in the publication of Japanese Laid-Open Patent No. 50-80799, it was not recognized that if the backlights of a transmissive type color liquid crystal device were replaced with a reflection plate, that was equivalent to saying a reflective type color liquid crystal device may be possible. Nevertheless, it is clear if actually test created, but with such a configuration it is dark and unusable. There are three causes: (1) ½ or more of the light is lost with the filter, (2) ⅔ or more of the light is further lost due to the color filter, and (3) the problems of parallax. The problems of parallax cannot be avoided with the TN (twisted nematic) mode and STN (super twisted nematic) mode in a transmissive liquid crystal device. The reason is because, since these modes necessarily use two polarizing plates, as long as the polarizing plates cannot be built into the cell, there occurs a gap that cannot be ignored between the reflective plate and the liquid crystal layer. The problems of parallax mentioned here are not only the problem of double reflection of the display as was the case with the conventional reflective type monochrome liquid crystal device, but it indicates a problem inherent in the reflective type color liquid crystal device.
The problems of parallax are explained using drawings. FIGS. 77(a) and (b) are cross section drawings of a reflective type color liquid crystal device using either the TN mode or the STN mode. This liquid crystal device is composed of an upper polarizing plate 7701, an upper glass substrate 7702, a liquid crystal layer 7703, a lower glass substrate 7704, a lower polarizing plate 7705, a light reflecting plate 7706, and a red-green-blue (RGB) tricolor filter 7707. Between the upper and lower glass substrates are additionally present a transparent electrode, an orientation film, and an insulating film, but they are omitted as they are not needed in explaining the problems of parallax. There are two problems of parallax. One of these is the mutual cancellation of the colors. In FIG. 77(a), the observer 7712 sees the reflected light 7711 emanating through the green filter, but this light is a blend of the introduced light 7713 passing through the red, green, and blue filters, and being scattered and reflected by the light reflecting plate. If the thickness of the lower glass substrate is sufficiently thick in comparison to the pitch of the color filters, the light passing through any colored filters will blend at equal probabilities. As a matter of fact, the light passing a course of red→green→blue, regardless of the wavelength of the light, is absorbed and becomes pitch black with any color filter, and only the light passing the course of green→green remains. Since the same can be said about the reflected light passing through a blue and a red filter, it becomes a problem that the brightness finally ends up ⅓ of that of a white display having no parallax. Another problem is that the color display becomes dark. FIG. 77(b) shows the status of a green display. Also, the part having applied a matrix-like hatching on the liquid crystal layer 7703 indicates that it is in the unlit status (dark status). The introduced light 7713 passes through red, green, and blue dots at equal probabilities, and ⅔ is absorbed by the red and blue dots being in the off status. Furthermore, after having been scattered and blended by the light reflection plate, ⅔ is again absorbed by the red and blue dots being in the off status, and the remainder reaches the observer 7712. Consequently, the green display becomes 1/9 the brightness of a white display minus the portion absorbed by the green filter, and becomes very dark. The use of the TN mode and the STN mode having such problems of parallax in a reflective type color liquid crystal device is very difficult.
Thus, in the past, there have been made attempts to obtain a bright reflective type color liquid crystal device by varying the liquid crystal mode. For example, in the article by Mr. Tatsuo UCHIDA, et al. (IEEE Transactions on Electron Devices, Vol. ED-33, No. 8, pp. 1207-1211 (1986)), the PCGH (phase change guest host) mode not requiring a polarizing plate was used upon having performed a comparison of the brightness of various liquid crystal modes in FIG. 2 of the report. Also, in the publication of Japanese Laid-Open Patent No. 5-241143, the PDLC (polymer distributed type liquid crystal) mode not requiring a polarizing plate was used in order to realize a reflective type color liquid crystal device. When a liquid crystal mode not requiring a polarizing plate is used, not only does the absorption of the light due to the polarizing plate disappear, there is also the benefit of being able to eliminate fundamentally the problems of parallax by providing a reflecting plate next to the liquid crystal layer. Nevertheless, on the other hand, the liquid crystal mode not requiring a polarizing plate has the problems that the contrast in general is low, and in particular the PCGH mode has hysteresis in the voltage transmissivity properties and intermediate tone displays are not possible. Also, the liquid crystal modes adding other substances into the liquid crystals have many problems in the aspect of reliability. Consequently, if the TN mode and STN mode are used, having been used widely from the past and showing satisfactory results, these have not been surpassed.
Also, there have been performed tests to obtain a bright reflective type color display using bright color filters. Generally, the color filters used in a transmissive type color liquid crystal device have spectral properties as shown in FIG. 78. The horizontal axis of FIG. 78 is the wavelength of the light, and the vertical axis is the transmissivity. 7801 is the spectrum of a red filter, 7802 is the spectrum of a green filter, and 7803 is the spectrum of a blue filter. The light that can be seen by a human has individual differences, but it is generally within the wavelength range of 380 nm to 780 nm, and in particular, the sensitivity is high within the range from 450 nm to 660 nm. All the color filters of FIG. 78 have wavelengths whereby the transmissivity becomes 10% or less in this range, and most of the light is rendered useless. Also, if the value having simply averaged the transmissivity in this range of wavelengths is defined as the average transmissivity, the average transmissivity of the red filter was 28%, the green filter was 33%, and the blue filter was 30%. For use in a reflective type liquid crystal device, brighter color filters are required. Thus, in the aforementioned article by Mr. Tatsuo UCHIDA, et al., it was proposed that by using bicolor filters having mutually complementing colors as shown in FIG. 8 of the report, it is brighter than the case of tricolor filters. Their spectral properties are shown in FIG. 79. The horizontal axis of FIG. 79 is the wavelength of the light, and the vertical axis is the reflectivity. 7901 is the spectrum of a green filter, and 7902 is the spectrum of a magenta filter. It is necessary to pay attention in the comparison because the vertical axis is displayed as reflectivity, but still, within the 450 nm to 660 nm wavelength range, both color filters have wavelengths whereby the transmissivity becomes 10% or less. The average transmissivity of the green filter was 41% and the magenta filter was 48%. Also, an article by Mr. Seiichi MITSUI, et al. (SID92 Digest, pp. 437-440 (1992)) also relates to a reflective type color liquid crystal device having used the same PCGH mode, but they use bright bicolor color filters such as in FIG. 2 of the report. The spectral properties are shown in FIG. 80. The horizontal axis of FIG. 80 is the wavelength of the light, and the vertical axis is the reflectivity. 8001 is the spectrum of the green filter, and 8002 is the spectrum of the magenta filter. The vertical axis displays reflectivity, but if the square root of the reflectivity at each wavelength is hypothesized as the transmissivity, at least the transmissivity of the green filter is smaller than 50% at wavelengths of 470 nm or less. The average transmissivity of the green filter was 68%, and the magenta filter was 67%. In both patent publications, there was no problem of parallax because a reflective plate is provided in a position near the liquid crystal layer, sandwiching a color filter. Consequently, because the light necessarily passes through the color filter two times, it is possible to secure sufficient coloration even when using such a bright color filter. Also, the color filter proposed in FIGS. 2(a), (b), and (c) of the previous publication of Japanese Laid-Open Patent No. 5-241143 is made brighter by using the three colors, yellow, cyan, magenta rather than the three colors, red, green, blue filter. Their spectral properties are shown in FIG. 81. The horizontal axis of FIG. 81 is the wavelength, and the vertical axis is the reflectivity. 8101 is the spectrum of the yellow, 8102 is the spectrum of the cyan filter, and 8103 is the spectrum of the magenta filter. The vertical axis is represented by reflectivity, and because there are no graduations on the axis, it is difficult to compare, but undoubtedly, within the range of 450 nm to 660 nm, all the color filters have wavelengths whereby the transmissivity is 10% or less. When roughly estimating the average transmissivity, the yellow filter was 0% the cyan filter was about 60%, and the magenta filter was about 50%.
Thus, the struggle of the conventional development of a reflective type color liquid crystal device was based on the starting point of trying to obtain a bright display by combining bright liquid crystal modes not using a polarizing plate and bright color filters. However, despite being bright color filters, the use of color filters having wavelengths whereby the transmissivity in the 450 nm to 660 nm wavelength range stopped at 10% was common.
The present invention aims to provide a reflective type color liquid crystal device that can display colors brighter and more brilliant than the prior art by using a liquid crystal mode that uses a polarizing plate such as TN mode and STN mode, having great merit while tackling the various problems of brightness and parallax, and to provide an electronic apparatus using this.